High temperature heat pumps have been used to upgrade low-grade thermal energy, such as that derived from air, soil, surface water or underground water, geothermal energy, solar energy, and industrial exhaust heat and process streams, to high-grade thermal energy via a thermodynamic cycle. A heat pump system has a compressor that imparts energy to the low-grade thermal stream. Heat pump systems use a working fluid, i.e., a refrigerant, to facilitate the generation and transfer of heat over the thermodynamic cycle. Heat pump systems have been used for both heating and cooling purposes.
Historically, chlorofluorocarbons such as trichlorofluoromethane (CFC-11), 1,1,2-trichlorotrifluoroethane (CFC-113) and 1,2-dichloro-1,1,2,2-tetrafluoroethane (CFC-114) were used as working fluids in heat pumps, refrigerators, and other heating/cooling devices and machines. Due to elevated levels of Ozone Depletion Potential (ODP) and Global Warming Potential (GMP) the foregoing working fluids exhibit, their use has largely ended.
Chlorofluorocarbons have been replaced in heating and cooling applications by other working fluids that exhibit lower ODP and GWP, such as hydrochlorofluorocarbons and hydrofluorocarbons. Such working fluids include chlorodifluoromethane (R-22), R-407C, R-410A, R-245fa, and 1,1,1,2-tetrafluoroethane (R-134a). R-407C is a blend of difluoromethane (R-32), 2-chloro-1,1,1,2-tetrafluoroethane (R-124), R-134a, 1-Chloro-1,1-difluoroethane (R142b). R-410A is a blend of R-22 and pentafluoroethane (R-125).
The replacement working fluids do not provide the same operating range in middle to high heating temperatures that chlorofluorocarbon working fluids do. Of particular interest are middle-high temperatures, i.e., condensing temperatures from 70° C. to 100° C. and high temperatures, i.e., condensing temperatures greater than 100° C. For instance, with R22, R407c and R410A, the highest condensing temperature is 65° C. For R134a, the highest condensing temperature attainable is 73° C. When condensing temperatures exceed the limit, cycle performance deteriorates and risk of accidents increase due to excessive discharge pressures and temperatures (from the compressor).
It would be desirable to have a working fluid that exhibits low ODP and GWP and provides excellent thermal performance in the middle and high temperature ranges, particularly in middle-high condensation temperature range of 70° C. to 100° C. or at the high condensation temperature range of greater than 100° C. It would be further desirable to have a working fluid that is useful in heat pump systems and other heating/cooling machines such as air-conditioning systems and chillers.